Understanding x-ray diffraction of nonpolar gallium nitride films

Abstract

X-ray diffraction (XRD) is widely used for the rapid evaluation of the structural quality of thin films. In order to determine how defect densities relate to XRD data, we investigated a series of heteroepitaxial nonpolar a-plane GaN films with different densities of dislocations and basal plane stacking faults (determined by transmission electron microscopy). Factors influencing XRD data include surface roughness effects, limited lateral coherence lengths, lateral microstrain, mosaic tilt, and wafer curvature, in addition to the defects present. No direct correlation between defect densities and any measured XRD parameter was found. However, the structural imperfections dominating XRD data can be identified by specific analysis of each individual broadening factor. This reductive approach permits full explanation of the in-plane rotational anisotropy of symmetric ω-scan widths for both a-plane and m-plane films: in these samples, mosaic tilt is the dominant factor.